The Brookhaven STEM (Scanning Transmission Electron Microscopy) Resource provides a unique service for mass and heavy atom cluster mapping of individual biological molecules. STEM I will continue localizing heavy atom clusters and mapping projected mass distribution, giving 1-2 nM useful spatial resolution and 2-10% mass resolution for objects as large as viruses imaged at low dose. STEM III is nearing completion with improved capabilities in resolution, specimen handling and data collection. We propose to continue to expand our productive user/collaborator program as site-specific labeling, specimen preparation, imaging and image analysis methods develop. Currently 65 projects are active in the areas of: protein and protein complexes, filaments and filamentous proteins, membranes and membrane proteins, DNA, RNA and nucleic acid/protein complexes, viral proteins and viruses, heavy atom cluster and cryo electron microscopy. All projects are expected to continue to lead to publications in the highest quality journals. Major emphasis of the core R&D program will continue to be quantitative understanding of specimen preparation as well as development of improved heavy atom cluster labels, improved microscopes, improved imaging techniques (including low dose elemental mapping using specific energy losses) and improved image analysis methods.